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POLAR FISH.
Term Paper ID:23943
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Essay Subject:
Examines extreme environment & need for & action of fishes' antifreeze glycopeptides.... More...
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6 Pages / 1350 Words
4 sources, 14 Citations,
MLA Format
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Paper Abstract:
Examines extreme environment & need for & action of fishes' antifreeze glycopeptides.
Paper Introduction:
This is an examination of the need and the mode of action of antifreeze glycopeptides found in polar fishes, especially the fishes of the Antarctic region. Continental drift working over geologic time periods caused the Antarctic continent to move from a temperate to a polar environment. The development of the Antarctic Convergence flowing along the edge of Antarctic waters created a thermal barrier which further isolated the local environment. The most important suborder of fishes inhabiting Antarctic waters, the Notothenioidei, underwent fundamental adaptations which allowed them not only to survive in the harsh environment, but eventually to prosper. The greatest danger to the fishes of supercooled waters like the Antarctic is the formation of ice. To counter the danger of lethal ice formation, the notothenioids have developed the ability to produce
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Although fish are cold-blooded and typically have the sametemperature as their environment, they can nonetheless survive even whentheir blood cools one full degree Celsius below its equilibrium freezingpoint, which, in practical terms, is the point at which ice crystals canform. The Antarctic Convergence isolated the fishes that were endemic tothe area and made it difficult for other fishes to penetrate the coolsouthern waters. Sodium chloride is also effective in lowering thefreezing point of fish fluids. A complex set of circumstances led to a cooling of theAntarctic waters. One of the important developments in this regard was theformation of a pattern of ocean currents known as the AntarcticConvergence. In addition,a third layer of ice crystals, known as anchor ice, grows along the bottomin areas where the water is no deeper than 3 meters. When the ice content of the water is below75%, the AFGPs increase their activity linearly as the concentration of icedecreases, reaching a maximal hysteresis of 1.19 degrees Celsius at aconcentration of 8.8% ice crystals. Acceptable sites can be found both on surfaces orientedexactly parallel to the prism plane as well as on surfaces branching atslight angles to that orientation. The Convergence, surrounding Antarctic waters between 5 and6 degrees south latitude, created a thermal barrier which prevented thearrival of warm currents from the north and at the same time inhibited themovement of temperate-water fishes into the area. "Adsorption of Ice ofFish Antifreeze Glycopeptides 7 and 8." Biophysical Journal 64 (January1993): 252-259. Beneath this surfaceice, large, elongated, loosely aggregated crystals of platelet ice take upanother meter or two of the water column beneath the sea ice. Drewes, Joel A. Low temperatures havebeen the rule in Antarctic waters for millions of years. The greatest danger to the fishesof supercooled waters like the Antarctic is the formation of ice. Tocounter the danger of lethal ice formation, the notothenioids havedeveloped the ability to produce antifreeze glycopeptides (AFGPs). Ice is dangerous for fishes because it so easily penetrates a fish'sgills and skin. The watertemperature around Antarctica is believed to have reached degrees Celsiusaround 1 to 15 million years ago, and the present more or less constantlow temperature (ranging from plus 3 degrees Celsius to minus 2 degreesCelsius) has been the norm for some 13 million years (Wohrmann 54). Furthermore Knight found that these AFGPs"inhibit crystal growth on prism planes and on surfaces close to thatorientation (Knight 253)." It appears from this research that AFGPmolecules need to find sites on which they fit in order to adsorb. Todaythe Antarctic waters spend four months of the year in darkness, and even inthe summer months, the sun's angle is so acute that less than one percentof the sunlight penetrates the waters. Joel Drewesand Kathy Rowlen have recently studied the possibility of a (-turn motifand concluded that the discovery that the lack of a solvent-exposedhydrophobic side to the (-turn structure "may imply that the mechanism bywhich AFGPs inhibit ice crystal growth does not depend on hydrophobicrepulsion of water from solution (Drewes 991)." Instead it may be thatcooperative hydrogen bonding of the AFGPs to the ice surface over thelength of a long peptide alters the surface of an ice crystal to such adegree that "water molecules can no longer order at the surface in a mannerthat would promote crystal growth (Drewes 991)." Other researchers have examined the activity of AFGPs 7 and 8,choosing to study the shorter chains because the longer chains present a"more complicated picture (Knight 257)." C.A. Today the suborderNotothenioidei dominates the Antarctic region. The notothenioidshave the highest degree of endemism and have radiated into the largestnumber of habitats (Bargelloni 854). While it is true that the waters of the Antarctic, like the waters ofthe North Polar Sea, are abnormally cool, it is not the temperature, butthe presence of multiple layers of ice in the water which causes thegreatest danger to the local marine fauna. "Evidence for a (-Turn Motif inAntifreeze Glycopeptides." Biophysical Journal 65 (September 1993): 985-991. Fossil remains found in the vicinity of Seymour Island in WestAntarctica and dating from approximately 38 million years ago demonstratethat the coastal waters there once supported several temperate-waterfishes. 1993): 47-65. The most important suborder of fishesinhabiting Antarctic waters, the Notothenioidei, underwent fundamentaladaptations which allowed them not only to survive in the harshenvironment, but eventually to prosper. Continental drift working over geologic time periodscaused the Antarctic continent to move from a temperate to a polarenvironment. show in theirresults that these two short AFGPs "bond onto ice prism planes alignedalong (-axes (Knight 253). Although most notothenioids begin asbottom dwellers, certain types, such as the Pleuragramma antarcticum, arefound at mid- and upper-levels (Wohrmann 122). Wohrmann, Andreas P.A. and Rowlen, Kathy L. "Antifreeze glycopeptides and peptides inAntarctic fish species from the Weddell Sea and the Lazarev Sea." MarineEcology Progress Series 13 (January 1996): 47-59.Studies in the Literary Imagination 26 (Apr. Thefive larger glycopeptides (AFGP 1-5) consist of a tripeptide repeat(alanine-alanine-threonine) with a disaccharide moiety attached to thethreonyl residues). Since thisjuxtaposition provides 24 hydrogen bonds between the ice and AFGP 8, and 24with AFGP 7, it is clear why chemical adsorption is essentiallyirreversible and why ice crystal growth is stopped completely. The antifreezes are either peptides (AFPs) or glycopeptides(AFGPs). There are at least eight different sizes of antifreeze glycopeptides,ranging in molecular weight from 26 Da to 34 Da (Wohrmann 48). The only reasonnotothenioids are not more in evidence probably lies more with the scarcityof resources than with anything else. Notothenioids, however, do not freezeuntil their temperature goes down to minus 2.2 degrees Celsius. This is an examination of the need and the mode of action ofantifreeze glycopeptides found in polar fishes, especially the fishes ofthe Antarctic region. Knight et al. But when the water is not clear, when ice is present in the wateraround them, cold water fishes begin to have a problem. Wohrmann, Andreas P.A. The mode of action of both the AFGPs and the AFPs is known to beadsorption, but the exact terms of this action are still being studied.Researchers have demonstrated that the antifreeze glycopeptides (AFGPs) caninhibit the growth of existing ice crystals, either from solution or frompure water, to temperatures in the neighborhood of 1.5 degrees Celsiusbelow the equilibrium freezing point, at concentrations of about 2 wt%.They prevent ice crystal growth in the fish down to temperatures as low asminus 2.2 degrees Celsius, which is 1.2 degrees Celsius below theequilibrium freezing point of the fish blood (Knight 252). But the number of species active in the Arctic Ocean is one andone-half times the number in Antarctic waters. All of theAntarctic fish, both notothenioids and non-notothenioids produce antifreezemolecules, although the non-notothenioids produce them in lesser amounts(Wohrmann 5 ). In McMurdoSound the average yearly water temperature is minus 1.87 degrees Celsius,the range varying only from minus 1.4 degrees to minus 2.15 degreesCelsius. These primarily demersalfish account for 9 percent of the fish of the region. The conditions of the Antarctic area are very harsh. In the Weddell Sea, the largest of the deep embayments of theAntarctic continent, the water temperature on the eastern shelf isconstantly below minus 1.8 degrees Celsius. "Antifreeze glycopeptides of the high-Antarctic silverfish Pleuragramma antarcticum (Notothenioidei)."Comparative Biochemical Physiology 111C (January 1995): 121-129. Knight, C. Works Cited Bargelloni, Luca; Ritchie, Peter A.; Patarnello, Tomaso; Battaglia,Bruno; Lambert, David M., and Meyer, Axel. The three smaller ones (AFGP 6-8) replace the firstalanine with proline (Wohrmann 47). The waters of the area remained relatively warm fora time. "Because the sites need to be severaltimes longer parallel to the alignment orientation than normal to it, theaxis of elongation of the location on the curved ice surface is expected tobe normal to the alignment direction (Knight 253)." In this research, twohydroxyl groups of each disaccharide get under the ice surface, which isthus shared by the crystal and by the disaccharide. In summer the average temperature rises to minus 1.8 degreesCelsius. There are more than 26 species of fishes in the watersaround Antarctica, which represents approximately 1% of the known fishes inthe world. "Molecular Evolution at SubzeroTemperatures: Mitochondrial and Nuclear Phylogenies of Fishes fromAntarctica (Suborder Notothenioidei), and the Evolution of AntifreezeGlycopeptides." Molecular Biology Evolution 11(6) (November 1994): 854-863. The exact determination of the conformation of antifreeze moleculesat the ice/water interface remains an experimental challenge. The AFGPs function by lowering the freezing point of the serum fluidsof the notothenioids. The AFGPs are periodic and come in a wide range of lengths(Knight 253). Theyachieve such a fit on the prism plane when it is aligned in one direction.Each adsorption site becomes a strip of prism-plane orientation elongatedin that direction. The development of the Antarctic Convergence flowing alongthe edge of Antarctic waters created a thermal barrier which furtherisolated the local environment. TheseAFGPs evidently work by adsorbing to the forming ice crystal, stopping itsdevelopment and preventing it from completing its formation. Most tropical andtemperate fishes freeze when their body fluids cool to approximately minuseight-tenths of a degree Celsius. A.; Driggers, E.; Devries, A.L. In thatenvironment, ice can rapidly form across the skin of fishes that have beenundercooled by only one-tenth of a degree Celsius. The need for antifreeze glycopeptides in Antarctic fishes originatesin the frigid temperatures of the Antarctic waters, low temperatures whichare in turn linked to the southward movement of the Antarctic landmass.The Antarctic Continent withdrew from the primary Gondwana mass some 5 to1 million years ago. Some time after the Seymour Island deposits were laid down,Antarctica separated completely from South America (from 3 to 23 millionyears ago). In essence, then, the antifreeze glycopeptides (AFGPs) adsorb to theforming ice crystal, shutting off ice crystal development and preventingfreezing of the fishes' body fluids. To combat this danger, the notothenioids produce an antifreezeglycopeptide (AFGP) which lowers the freezing point of their body fluidsand allows them to overcome the environmental challenge. Two to three meters of annualsea ice cover the water ten months out of the year.
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